Electronically controlled radio frequency (RF) switches, or transmission gates, are commonly used in a wide variety of RF systems. Applications include adaptive tuning of filters or resonant circuits using reactive components or transmit/receive (T/R) switches to allow an antenna to be connected to various transmitters or receivers.
RF switches are designed to pass RF signals while in an on state, and block RF signals while in an off state. RF switches are controlled by a direct current (DC) signal or signals. Some RF switch technologies include PIN diodes, conventional electromechanical relays, micro-electromechanical systems (MEMS) relays, and field-effect transistors (FETs). Each of these technologies offer several advantages and disadvantages.
PIN diode RF switches are advantageous in that they are all-electronic, fast, and are high RF power-capable. However, they are expensive and require high drive power, complex control, and complex bias networks to separate control and RF.
Conventional relays are robust, very high RF power-capable, simple to control, do not require bias networks, and have low RF parasitics. However, they are slow, expensive, physically large, and having a limited operational life.
MEMS relays are small in size, have low RF parasitics, and can be integrated. However, they have limited RF power capability, poor robustness to electrical stress, require complex control, have unusual control signal requirements (voltage and current), are expensive, and have a limited lifetime.
FETs are small and can be easily integrated, require low control power, have fast operation, are all-electronic, and are high RF power-capable. However, they may require bias networks to separate control and RF, and have a relatively high off-state capacitance (low isolation) and non-linear off-state capacitance (harmonic generation, tuning difficulty).
An FET RF switch that overcomes some of the disadvantages of the FET RF switches currently available would be highly desirable. Accordingly, a solution to the problem of non-linear capacitance and tuning difficulties in currently available FET RF switches is needed.